This invention relates to silica sols and to processes for preparing the surface of polymeric articles with such sols. More particularly this invention is concerned with colloidal suspensions of very fine particles in aqueous medium of polysilicic acid in a particular aggregate which demonstrates unique antistatic and soil resistant properties when applied as a uniform and continuous coating on polymeric surfaces.
Conventional antistatic agents are usually viscous liquids or greases in their native state. Application of these agents on polymeric surfaces or on other materials may severely restrict subsequent processing of these products and limit their available end uses.
Antistatic and soil resistant characteristics may also be obtained by coating or otherwise treating articles with silica sols. This is known particularly in the manufacture and use of film products and textile fibers. These sols may be prepared by various methods the result of which is a colloidal system of what has been described as polymeric silicic acid sol. These silicic acid sols are relatively unstable due to high surface area and low average particle diameter below 5 millimicrons. To produce silica sols of long-term stability requires control of particle size and pH and the elimination of impurities.
The prior art has been concerned primarily with the development and use of stable sols highly concentrated in silica. U.S. Pat. No. 3,053,662 to Mackey et al., for example, discloses a photographic film having a coating thereon containing gelatin and an antistatic agent selected from the group consisting of hydrated silica and polymerized silicic acid. These forms of silica are faintly opalescent aqueous sols and contain between 15 and 30% colloidal silica as SiO.sub.2. The diameter of the silica particles ranges from 7 to 15 millimicrons (m.mu.), i.e., 70 to 150 angstroms.
U.S. Pat. No. 2,440,711 to Bechtold teaches the formation of polysilicic acid (hereinafter PSA) by reacting ethyl orthosilicate, ethanol and hydrochloric acid to obtain a solution containing about 15% PSA on an SiO.sub.2 basis. This solution, when combined with other materials and applied as a coating, is known to improve the scratch resistance of certain polymeric surfaces such as methyl methacrylate. When this PSA solution is diluted and applied on film surfaces as a coating, improvements in the surface properties may be obtained including improved static prevention. A problem may arise, however, because the adhesion level of the PSA coating to the film substrate may be very poor.